| Shield Platform Orogen Basin Large igneous province Extended crust | Oceanic crust: 0–20 Ma 20–65 Ma >65 Ma |
In geology, a crust is the outermost solid shell of a rocky planet or moon, which is chemically distinct from the underlying mantle. The crusts of Earth, our Moon, Mercury, Venus, Mars, Io, and other planetary bodies have been generated largely by igneous processes, and these crusts are richer in incompatible elements than their respective mantles.
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The crust of the Earth is composed of a great variety of igneous, metamorphic, and sedimentary rocks. The crust is underlain by the mantle. The upper part of the mantle is composed mostly of peridotite, a rock denser than rocks common in the overlying crust. The boundary between the crust and mantle is conventionally placed at the Mohorovičić discontinuity, a boundary defined by a contrast in seismic velocity. Earth's crust occupies less than 1% of Earth's volume.
The oceanic crust of the Earth is different from its continental crust. The oceanic crust is 5 km (3 mi) to 10 km (6 mi) thick[1] and is composed primarily of basalt, diabase, and gabbro. The continental crust is typically from 30 km (20 mi) to 50 km (30 mi) thick, and is mostly composed of slightly less dense rocks than those of the oceanic crust. Some of these less dense rocks, such as granite, are common in the continental crust but rare to absent in the oceanic crust. Both the continental and oceanic crust "float" on the mantle. Because the continental crust is thicker, it extends both above and below the oceanic crust, much like a large iceberg floating next to smaller one. (The slightly lighter density of felsic continental rock compared to basaltic ocean rock also contributes to the higher relative elevation of the top of the continental crust.) Because the top of the continental crust is above that of the oceanic, water runs off the continents and collects above the oceanic crust. The continental crust and the oceanic crust are sometimes called sial and sima respectively. Due to the change in velocity of seismic waves it is believed that on continents at a certain depth sial becomes close in its physical properties to sima and the dividing line is called Conrad discontinuity.
The temperature of the crust increases with depth, reaching values typically in the range from about 200°C (392°F) to 400°C (752°F) at the boundary with the underlying mantle. The crust and underlying relatively rigid mantle make up the lithosphere. Because of convection in the underlying plastic (although non-molten) upper mantle and asthenosphere, the lithosphere is broken into tectonic plates that move. The temperature increases by as much as 30°C (about 50°F) for every kilometer locally in the upper part of the crust, but the geothermal gradient is smaller in deeper crust. [2]
Partly by analogy to what is known about our Moon, Earth is considered to have differentiated from an aggregate of planetesimals into its core, mantle and crust within about 100 million years of the formation of the planet, 4.6 billion years ago. The primordial crust was very thin, and was probably recycled by much more vigorous plate tectonics and destroyed by significant asteroid impacts, which were much more common in the early stages of the solar system.
The Earth has probably always had some form of basaltic crust, but the age of the oldest oceanic crust today is only about 200 million years. In contrast, the bulk of the continental crust is much older. The oldest continental crustal rocks on Earth have ages in the range from about 3.7 to 4.28 billion years [3][4] and have been found in the Narryer Gneiss Terrane in Western Australia, in the Acasta Gneiss in the Northwest Territories on the Canadian Shield, and on other cratonic regions such as those on the Fennoscandian Shield. A few zircons with ages as great as 4.3 billion years have been found in the Narryer Gneiss Terrane.
The average age of the current Earth's continental crust has been estimated to be about 2.0 billion years.[5] Most crustal rocks formed before 2.5 billion years ago are located in cratons. Such old continental crust and the underlying mantle lithosphere are less dense than elsewhere in the earth and so are not readily destroyed by subduction. Formation of new continental crust is linked to periods of intense orogeny or mountain building; these periods coincide with the formation of the supercontinents such as Rodinia, Pangaea and Gondwana. The crust forms in part by aggregation of island arcs including granite and metamorphic fold belts, and it is preserved in part by depletion of the underlying mantle to form buoyant lithospheric mantle.
The continental crust has an average composition similar to that of the igneous rock, andesite. The composition tabulated below and the following discussion are based largely on the summary by Rudnick and Gao (2003).[6] Continental crust is enriched in incompatible elements compared to the basaltic ocean crust and much enriched compared to the underlying mantle. Although the continental crust comprises only about 0.6 weight percent of the silicate Earth, it contains 20% to 70% of the incompatible elements.
| Oxide | Percent |
|---|---|
| SiO2 | 60.6 |
| Al2O3 | 15.9 |
| CaO | 6.4 |
| MgO | 4.7 |
| Na2O | 3.1 |
| Fe as FeO | 6.7 |
| K2O | 1.8 |
| TiO2 | 0.7 |
| P2O5 | 0.1 |
All the other constituents except water occur only in very small quantities, and total less than 1%. Estimates of average density for the upper crust range between 2.69 g/cm3 and 2.74 g/cm3 and for lower crust between 3.0 g/cm3 and 3.25 g/cm3[7].
A particularly large meteorite is thought to have collided with the forming Earth, and part of the material ejected into space by the collision accreted to form the Moon. As the Moon formed, the outer part of it is thought to have been molten, a “lunar magma ocean.” Plagioclase feldspar crystallized in large amounts from this magma ocean and floated towards the surface. The cumulate rocks form much of the crust. The upper part of the crust probably averages about 88% plagioclase (near the lower limit of 90% defined for anorthosite): the lower part of the crust may contain a higher percent of ferromagnesian minerals such as the pyroxenes and olivine, but even that lower part probably averages about 78% plagioclase.[8] The underlying mantle is denser and is also olivine-rich.
The thickness of the crust ranges between about 20 and 120 km. Crust on the far side of the moon averages about 12 km thicker than that on the near side. Estimates of average thickness fall in the range from about 50 to 60 km. Most of this plagioclase-rich crust formed shortly after formation of the moon, between about 4.5 and 4.3 billion years ago. Perhaps 10% or less of the crust consists of igneous rock added after the formation of the initial plagioclase-rich material. The best-characterized and most voluminous of these later additions are the mare basalts formed between about 3.9 and 3.2 billion years ago. Minor volcanism continued after 3.2 billion years, perhaps as recently as 1 billion years ago. There is no evidence of crustal formation or deformation due to plate tectonics.
Study of the Moon has established that a crust can form on a rocky planetary body significantly smaller than Earth. Although the radius of the Moon is only about a quarter that of Earth, the lunar crust has a significantly greater average thickness. This relatively thick crust formed almost immediately after formation of the Moon. Magmatism continued after the period of intense meteorite impacts ended about 3.9 billion years ago, but igneous rocks younger than 3.9 billion years make up only a minor part of the crust.[9]
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[[File:|thumb|230px|Earth cutaway from the core to the exosphere.]] The crust is the outer hard layer of the Earth. The crust is a part of the lithosphere.
The crust is made up of different types of rocks; igneous, metamorphic, and sedimentary rocks.
Below the crust is the mantle. The upper part of the mantle is made up of peridotite, a rock denser than rocks common in the crust. The crust and the upper mantle make up the lithosphere. The lithosphere is broken up into tectonic plates that can move. The Earth's crust is less than 1% of Earth's volume.
The crust has two different parts. One is the continental crust (under the land) and the other is oceanic crust (under the ocean). The continental crust is thicker, and the oceanic crust is thinner. Thicknesses of the crust can be anywhere from 5-70 km.
The temperature of the crust increases with depth. Where the crust meets the mantle the temperatures can be between 200 °C (392 °F) to 400 °C (752 °F). [1]
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About 75% of the Earth's crust is composed of two elements, oxygen and silicon. These usually occur in combination with common metals such as aluminium and iron. The figures are: [2]p41
| Oxygen | 46.6 |
| Silica | 27.7 |
| Aluminium | 8.1 |
| Iron | 5.0 |
| All others | 12.6 |
A single family of silicates, the feldspars, account for about half of the material in the crust (60% by weight). and quartz is a sizeable proportion of the rest. Other common minerals are mica and hornblende.
Only 8% of the Earth's crust is non-silicate minerals, and this includes carbonates, sulfides, chlorides and oxides.
Igneous rocks make up over 90% of the Earth's crust by volume.[2]p47 This is not noticed because they are mostly covered by sedimentary and metamorphic rocks.
The Earth's mantle and crust formed about 100 million years after the formation of the planet, about 4.6 billion years ago. At first the crust was very thin, and was probably changed often as the tectonic plates shifted around a lot more than they do now. The crust would have been destroyed many times by asteroids hitting the Earth, which were much more common in the Late Heavy Bombardment.
The oldest oceanic basalt crust today is only about 200 million years. Most of the continental crust is much older. The oldest continental crustal rocks on Earth are between 3.7 to 4.28 billion years old. [3][4] These have been found in the Narryer Gneiss Terrane in Western Australia, in the Acasta Gneiss in the Northwest Territories on the Canadian Shield, and on the Fennoscandian Shield. A few zircons at least 4.3 billion years old have been found in the Narryer Gneiss Terrane.
The average age of the current Earth's continental crust has been estimated to be about 2.0 billion years.[5] Most crustal rocks formed before 2.5 billion years ago are in cratons. Such old continental crust and the mantle below it are less dense than other places in the earth. These are not easily destroyed when the plates shift. The making of new continental crust is linked to times of major orogeny or mountain building. This happens at the same time as the formation of the supercontinents such as Rodinia, Pangaea and Gondwana. The crust forms in part by the coming together of island arcs including granite and metamorphic fold belts. They are kept together partly by the using up of the mantle below the crust, which makes a mantle on which the crust can float.
   | USGS Crust Thickness Map - The Earth's Crust |
 | Structure and composition of the Earth - Structure and composition of the Earth - Australian Museum |
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